Curable dental restorative materials containing a particulate filler are known. Generic dental restorative materials have to meet a number of requirements. First and foremost, the toxicity of a dental restorative material must be low to be suitable for use in the oral environment. Moreover, mechanical properties such as strength and abrasion resistance must be high in a cured dental material for applications where mastication forces represent the primary challenge to the performance of the dental restorative material. Good aesthetic properties such as luster and translucency are required in applications where the location of a dental restoration is visible.
The presence of a particulate filler in a dental restorative material increases the mechanical strength and improves the surface properties of the cured dental material. Specifically, a particulate filler having a median particle size (D50) in the range of 0.05 to 5 μm provides good surface properties and mechanical properties. However, in case the particulate dental filler contains particles having a diameter in the range of the wavelength of visible light (400 to 800 nm), the luster and aesthetic character of the dental restorative material deteriorate the more the refractive index of the filler deviates from the refractive index of the cured resin matrix.
In dental restorative materials such as dental composites containing a curable resin, resin shrinkage upon polymerization in the curing process tends to cause gap formation between the restorative composite and the tooth. As a consequence micro leakage, secondary caries and decreased longevity of the restoration represents a problem with prior dental restorative materials. In order to alleviate the shrinking problem and to reinforce dental restorative materials, a high filler load is preferred.
Accordingly, spherical composite particles containing inorganic particles in an organic matrix were suggested previously in order to improve the aesthetic properties while maintaining good mechanical properties and while avoiding the shrinking problem.
EP2604247 discloses a process for the preparation of composite filler particles, comprising: coating a particulate filler having a median particle size (D50) of from 1 to 1200 nm with a coating composition containing a film-forming agent, agglomerating the coated particulate filler, whereby the agglomeration is carried out by spray agglomeration or growth agglomeration, and crosslinking the granulation of the coated particulate filler for providing composite filler particles having a median particle size (D50) of from 1 to 70 μm, wherein the particulate filler is the main component by volume of the composite filler particles.
On the other hand, filler particles prepared in a conventional sol-gel method are known. According to the sol-gel method, spherical silica particles of high mono-dispersion and small average particle size may be produced by subjecting an alkoxide of silicon to hydrolysis and polycondensation. By using an alkoxide of silicon and an alkoxide of a metal other than silicon as raw materials, silica-based composite oxide particles can be produced by the sol-gel processing. Previously, particles of silica-titania, silica-alumina, or silica-zirconia were disclosed as the silica-based composite oxide particles. The silica-based composite oxide particles produced by the sol-gel processing are provided with characteristic properties depending upon the kind of the metal oxide contained in the particles together with silica. For example, by changing the mixing ratio of silica and a metal oxide other than silica, the refractive index of the particles may be adjusted while the optical transparency of the particles is maintained. Accordingly, a transparent composite resin or a dental composite resin may be obtained wherein the refractive index of the particles is similar to the refractive index of the polymerizable matrix.
Moreover, it is necessary that the particulate dental filler composition has radioopacity in addition to the transparency. Accordingly, by controlling the mixing ratio of a metal oxide to make the refractive index of filler identical with that of the cured resin, there can be obtained a dental filler having both transparency and radioopacity.
However, silica-based composite oxide particles formed by sol-gel processing tend to coagulate and to form aggregates of primary particles. Moreover, the diameters of the particles formed become non-uniform over time.
WO2010/045105 discloses fillers containing silica-zirconia nanoclusters. The fillers are prepared by mixing a sol of silica nanoparticles with a sol of preformed, crystalline nanozirconia particles. Subsequently, the mixture is heated to a temperature from about 400° C. to about 1000° C. According to WO2010/045105, the milling of the heated mixture is essential to form a filler comprising silica-zirconia nanoclusters.
EP2463235 discloses a method for producing silica-zirconia composite oxide particles each coated with a silica layer. Accordingly, a liquid dispersion of silica-zirconia composite oxide particles is obtained by reacting an alkoxide of silicon with an alkoxide of zirconium in a water-containing solvent that contains acetonitrile so that the reaction liquid is to contain not less than 10% by mass of acetonitrile. Subsequently, the surface of each silica-zirconia composite oxide particle is coated with a silica layer by reacting the silica-zirconia composite oxide particles dispersed in the liquid dispersion with an alkoxide of silicon and/or a condensable compound derived from the alkoxide in the liquid dispersion of silica-zirconia composite oxide particles. The presence of a substantial amount of acetonitrile is essential for preventing the particles from coagulating.
Therefore, it is difficult to provide silica-based composite oxide particles of high mono-dispersion and uniform particle diameter by the sol-gel method so that a high filler load in a dental composition is difficult to attain.
U.S. Pat. No. 8,658,188 discloses a nanoparticulate bioactive radio-opaque material for use in dental compositions, which comprises a matrix containing oxides of Si, Ca, Na and optionally P. A radiopacifier embedded in the matrix is selected from the group consisting of metals, metal oxides and metal salts (such as phosphates, halogenides, sulphates) of elements with an atomic mass greater than 20, preferably greater than 85. The particles are prepared by flame pyrolysis at a high flame temperature of up to 2600° C. and are obtained as aggregates having a large BET surface of at least 30 m2/g. Due to the low bulk density and snow-like behavior, compaction of the particles is necessary prior to incorporation into a dental composition. However, the aggregates of nanoparticles increase the viscosity of a dental composition to an extent which is not useful in practice.
U.S. Pat. No. 4,503,169 discloses radiopaque, low visual opacity dental composites and non-vitreous microparticles for use therein, the microparticles individually containing amorphous silica microregions interspersed with radiopacifying polycrystalline ceramic metal oxide microregions. U.S. Pat. No. 6,030,606 discloses dental restoratives.